Tobacco curing and drying basically involves three phases or stages. First, there is the curing or yellowing stage where the tobacco leaf material is treated and conditioned prior to leaf drying. After curing or yellowing, the tobacco leaf is subjected to a drying period. Once the leaf has been dried then the final stage of the process entails drying the stems of the leaf material.
In order to produce high quality tobacco it is absolutely essential that the tobacco farmer carry out an effective curing or yellowing process. During this part of the curing and drying process, the rate that moisture is removed from the leaf is very critical. Prior to being placed in a tobacco curing and drying structure, the tobacco leaf includes a very substantial moisture content and as a part of curing and drying, the process entails continuously and systematically removing moisture from the tobacco leaf over an extended period of time. The tobacco leaf includes what is referred to as stomata and the moisture held within the leaf escapes or migrates through the stomata of the leaf. It has long been known and appreciated that the appropriate curing and drying rate of tobacco is controlled by maintaining a certain relationship between dry bulb and wet bulb conditions within the curing and drying structure. Dry bulb temperature is defined as the temperature of the air within the curing and drying structure during the process. Wet bulb temperature is defined as the temperature of a thermometer within the same curing and drying structure with the thermometer being embedded within a wet wick. During the curing and drying process, forced air is moved by the tobacco leaf material and during the process a moisture gradient is established between the leaf and the air and consequently, moisture flows from the leaf to the air. As additional moisture is added to the air, the wet bulb temperature of the air increases. It follows that if sufficient moisture is not removed from the air, that the wet bulb temperature will continue to rise until an equilibrium stage is realized and no further moisture is removed from the leaf.
In order to maintain an appropriate environment for continually removing moisture from the tobacco leaf, it becomes necessary throughout the process to continuously mix fresh outside air, which contains less moisture, with the air inside the tobacco curing and drying structure. During this process a certain quantity of high humidity air will be forced out the exhaust dampers of the curing and drying structure. This will have the effect of reducing the wet bulb temperature within the structure. It then follows that when the amount of moisture removed from the structure is equal to the amount of moisture removed from the tobacco leaf material then the wet bulb temperature will remain constant and ideal curing conditions will exist.
In the past, many farmers have set the inlet and exhaust dampers of the curing and drying structure manually. Past results of farmers indicate that it is virtually impossible to precisely manually set the dampers of a curing and drying structure. The result is that manual damper settings create an imbalance between leaf moisture loss and the structure moisture loss. This of course means that the wet bulb temperature will vary over a period of time and will deviate from a desired set point.
It has been known to automatically control the dampers of a tobacco curing and drying structure and to attempt to maintain a proper relationship between wet bulb and dry bulb temperatures within a curing and drying structure. In fact, it is known to utilize a three-mode controller for controlling the damper position of the inlet to the curing and drying structure. However, the real problem presented with a three-mode controller is that the conditions within the structure and the conditions outside the structure are continuously changing. This is a problem because a conventional three-mode controller is set based on conditions within the structure and conditions outside of the structure remaining constant. It is certainly appreciated that the conditions outside the curing and drying structure change and consequently, the fresh outside air that is brought into the structure will vary in moisture content from time to time. By the same token, the leaf conditions change throughout the cure or throughout the process and that in turn effects a change in the internal environment. Consequently, a conventional three-mode controller attempts to maintain a certain wet bulb condition within the curing and drying structure based on continuously changing internal and external conditions. The net result is that it is virtually impossible to continuously accurately control the wet bulb conditions within a tobacco curing and drying structure without continuously changing the control parameters relative to the inside and outside conditions.
Therefore, there is and continues to be a need in the tobacco curing and drying field and in the fields of curing and drying other related agricultural products, for a control system that continuously monitors inside and outside conditions and varies key control parameters such that they respond to such changing conditions to effectuate a precise controlled environment.